Hybrid vehicles are vehicles in which two power sources, for example an internal combustion engine and an electric motor, play a part in supplying power for use in automotive propulsion. In some hybrid vehicles, both the engine and the motor are connectable to driveline components for supplying rotary power to wheels for driving the vehicle. In other hybrid vehicles, only the motor is connectable to the driveline components for supplying the rotary power; and the engine is used only as a prime mover for driving a generator which generates the electrical power needed to operate the motor. Examples of existing hybrid vehicles include passenger cars (sometimes referred to as “automobiles”), vans, buses and light trucks. There is also an increasing interest in incorporating hybrid vehicle technology into light rail vehicles such as trams.
In nearly all hybrid vehicles, it is desirable to minimise the size and weight of the automotive components. In passenger cars, successfully minimising the size and weight of these components results in greater room and hence comfort for the passengers, and may also improve the operating efficiency of the vehicle. The same is true in other hybrid vehicles.
At the same time, it is also desirable to maximise the power output of electrical machines used in hybrid vehicles: consumers are used to the high power output of internal combustion engines and so would be disappointed with lesser performance from hybrid vehicles. This is true of both motors and generators used in hybrid vehicles. For motors, a high mechanical power output is needed in order to provide adequate automotive propulsion; and accordingly for generators, a high electrical power output is needed in order to operate the motors at high power.
Thus, for hybrid vehicles it is desirable to use electrical machines with high power output per unit mass (often referred to as “power density” and measured in kW/kg). Unfortunately, however, existing electrical machines do not satisfy these requirements and so do not lend themselves well to use in hybrid vehicles.
Similar considerations apply to electrical machines used to power purely electric vehicles.
Electrical machines that are of the axial-flux type are in several ways suited for use in hybrid vehicles and purely electric vehicles. One reason for this is because they can be designed to have a high power density. However, the design of axial-flux machines insofar as it relates to their assembly and operation is not optimised: assembly can be difficult and operation can be unreliable. This is particularly the case in relation to the rotors of such machines. An object of this invention is to address this problem.